The present invention relates to a process for improving heat transfer in flowing fluids by introduction of the fluid tangential to the internal wall of a heat exchanger. The invention further relates to heat exchanger structure designed to cause tangential introduction of the flowing fluid.
A common problem in processing fluids, particularly gases, is the heating of the fluid by transfer of heat from the surrounding equipment. A reduction in the overall resistance between a heat source and a heat sink for a given thermal load, as measured by an increase in the heat transfer coefficient, will result in a smaller heat exchanger, and therefore a lower equipment cost. Alternately, a reduction in the resistance to heat transfer will result in more efficient operation of the heat exchanger and the ability to process more fluid or to increase the temperature of the exit fluid without increasing the energy expended. It is known that an increase in the flow velocities of the fluid under fully developed turbulent flow will increase the heat transfer coefficients. However, pressures generated by increased flow increase at a much faster rate. Consequently, heat transfer rates per unit of pump power will actually decrease with the increased velocity.
Another approach to enhancing the heat transfer coefficients is to alter the hydrodynamic characteristics of the fluid flowing through the system by modifying the surface characteristics or configuration. This can be accomplished by passive means, such as surface roughness or the placement of fins or straight or twisted tape inserts, or active means, such as fluid oscillation, surface vibration, injection or suction at the heat transfer surface or the addition or generation of a second phase in the flowing stream.
Experimentation has shown that a maximum of about twenty percent increase in heat transfer at a constant pumping power basis can be generated by use of twisted tapes in the fluid flow path. For Reynolds numbers from 5,000 to 30,000 heat transfer can be enhanced 40% to 200%. However, the friction factor increases to between 160% and 1110%. Thus, the loss in pumping power exceeds the gain in heat transfer, resulting in a net decrease in heat transfer at constant pumping power. Additionally, use of twisted tapes and related devices are only effective at lower Reynolds numbers. Surface vibration or fluid oscillation has also shown about a 200% increase in heat transfer but only at low velocities and the technique requires complex equipment and the supply of and additional external power source.
A fourth method of enhancing heat transfer is to create a swirling motion in the fluid flowing through the heat exchanger. Results in tubular heat exchangers show that heat transfer and friction factors significantly decay in a distance from the inlet equivalent to 20 tube diameters and that losses due to friction increase at a rate greater than the increase in heat transfer. Injection induced swirl on single phase heat transfer has been shown to increase the heat transfer coefficient 6 fold at a momentum ratio of about 9.6. However, on a constant pumping basis this results in only about a 20% enhancement of heat transfer.
In a modified procedure to create swirling motion, it was proposed that a portion of the fluid be injected tangentially while additional fluid is injected axially. It was theorized that the swirling flow created would cause the hotter fluid near the wall to move toward the center, thus resulting in a thinning of the thermal boundary layer and an increase in the heat transfer (Kreith and Margolis, "Heat Transfer and Friction in Turbulent Vortex Flow", Applied Scientific Research, Vol. 8, 1959, p. 457-473). However, they never tested this concept. Weede & Dhir ("Critical Heat Flux Enhancement Using Long Tangential Flow Injection", Nuclear Technology/Fusion. Vol. 4 (Sept. 1983, pp. 483-488) demonstrated, on a subcooled fluid (Freon-113) a net enhancement, at a constant pumping basis, of 40%. However, to do so fluid had to be injected at several locations along the heat exchanger tube.
Dhir et al ("Enhancement of Forced Convection Heat Transfer using Single and Multi-stage Tangential Injection", ASME HTD. Vol. 119. (Dec. 1989) have reported on experimentally determined enhancement of heat transfer with air as the test fluid and Reynolds numbers between 15,000 and 58,000. The air was injected tangential to the inner walls of the heat exchanger tubes through square edged injectors extending perpendicular from the tube surface. The net enhancement of heat transfer, at constant pumping power, was between 3% and 14% depending on the momentum ratio. It was also found that the effectiveness of the system is highly dependent on the ratio of the rates of tangential to total momentum fluxes.
While various different techniques have been theorized or demonstrated to increase heat transfer, the frictional effects and other countervailing forces limit the capacity to increase heat transfer at the same pumping power. These alternate techniques may also require substantial changes to the heat exchanger equipment and/or the supplying of additional energy to the system.
Thus there is a need for a simple method to significantly increase heat transfer without a substantial modification of the heat exchange equipment or the addition of substantial pumping energy to move the fluid through the system. There is a further need for a simple equipment modification which will allow a substantial increase in heat transfer without increasing the energy to pump the fluid and which will not significantly decrease the flow of fluid through the heat exchanger.